Conventionally, a time compression multiplex (TCM) optical communication system has been used for a bothway communication system using a single optical line. In that system, an upward direction signal and a downward direction signal are forwarded to an optical cable alternatively, so that the system has been used for low rate communication up to 28 Mbits/second.
When high rate communication is requested in a system with a single optical cable, for instance, the high rate communication is higher than 50 Mbits/second, a wavelength division multiplex (WDM) optical communication system has been used. That system uses two wavelength bands, for instance 1.3 .mu.m band and 1.5 .mu.m band for an upward direction and a downward direction, respectively.
FIG. 1A shows a block diagram of a prior bothway wavelength division multiplex (WDM) optical communication system. In the figure, the numeral 70 is a single optical fiber cable coupling a pair of terminal stations 71 and 72. Each terminal station has an optical transmitter 73 which outputs a light signal through a laser, an optical receiver 74 which receives an optical signal from an opposite side, and a multiplexer/demultiplexer 75 having an output port coupled with the single optical fiber cable 70 and a pair of input ports coupled with the transmitter 73 and the receiver 74 so that the light from the transmitter 73 is forwarded to the optical cable 70 and the light from the line 70 is forwarded to the receiver 74. The wavelength of the output of the transmitter 73 differs from that on the other side, for instance, the wavelength in one direction is 1.3 .mu.m band, and the wavelength in the other direction is 1.5 .mu.m band. Those wavelengths of 1.3 .mu.m band and 1.5 .mu.m band are multiplexed in the optical cable 70.
However, the system of FIG. 1A has the disadvantage that a laser in the transmitter 73 must provide an oscillation wavelength which coincides with a center of a passband of the multiplexer/demultiplexer. However, it should be noted that the oscillation wavelength of a laser depends upon ambient temperature, bias current and/or producing error, and therefore, it is rather difficult to obtain a laser with the requested accurate oscillation wavelength. Further, the difference between wavelengths in two directions must be large enough for suppressing cross talk between the two wavelengths, and therefore, two kinds of lasers through different producing processes must be used for a large oscillation wavelength difference, for instance, 1.3 .mu.m and 1.5 .mu.m. Therefore, producing yield rate of a laser is rather low, and so, the cost of the communication system of FIG. 1A is rather high.
FIG. 1B shows another prior bothway wavelength division multiplex optical communication system, which uses the common single wavelength band in both directions. The system of FIG. 1B has not existed in the market, but we considered it in our research. In the figure, the numeral 80 is an optical cable having a pair of ends each coupled with terminal stations 81 and 82. Each of the terminal stations has an optical transmitter 83, an optical receiver 84 for receiving light from the other side, and a directional coupler 85 coupled with an end of the optical cable 80, an output of the optical transmitter 83, and an input of the optical receiver 84, so that the light from the transmitter 83 is applied to the optical cable 80, and the light from the optical cable 80 is applied to the optical receiver 84.
However, the system is FIG. 1B has the disadvantage that the receiver 84 receives not only the light from the optical cable 80 but also the light from the transmitter 83 in the same terminal station through leakage in the directional coupler 85, and therefore, the signal characteristics are deteriorated, although the lasers in the transmitters in each stations oscillate with the same wavelength as each other.